Need a simple calculator during editing a file using vim and don’t want to click away from it? Then first add the following to your ~/.vimrc:

In order to use the calculator  while you are editing some file using vim, hit CTRL-C in the insert mode and then enter what you want to calculate (e.g., 2+3 or 4.5*7.9).

(It) first calls the built-in input() function to request the user to type in their calculation, which input() then returns as a string. That input string is then passed to the built-in eval(), which evaluates it as a Vimscript expression and returns the result. Next, the built-in string() function converts the numeric result back to a string, which the key-mapping’s <C-R>= sequence is then able to insert.

Reference: here.